In an LSI manufacturing process, electrification of a wafer is a big problem, and it is urgently required to establish a technology for prevention of electrification.
Description is hereunder made for troubles caused by electrification of a wafer, as an example of an electrified object.
As a wafer is generally handled with insulating fluororesin or quartz for preventing it from being contaminated, a very high electric potential is charged when it contacts something during handling. Results of measurement of electric potential in electrified wafers are shown in Table 1 below.
TABLE 1 ______________________________________ Electrical potential in an electrified wafer ______________________________________ When handled by teflon forceps . . . +500 V.about.+3300 V or more When put on a polypropylene stand . . . +600 V.about.+2000 V When a wafer is put on a quartz plate with teflon forceps . . . +1000 V.about.+1500 V ______________________________________
A range of measurement by an electrometer =-3300 V.about.+3300 V.
As shown by this result, it has turned out that, when a silicon wafer is handled by resin materials or quartz, always positive electricity is charged in the wafer because of the electrification column, and also that the electric potential is fairly high.
Also it has turned out that, when a wafer is electrified, two types of trouble as shown below are caused and it is a big cause for substantial decrease in yield in a semiconductor manufacturing process;
1 Adhesion of airborne particles due to electrostatic force. PA1 2 Breakage of a device due to discharge of static electricity. PA1 1 Generating ions by means of the corona discharge method and neutralizing an electrified wafer with the ions. PA1 2 Neutralizing an electric charge in a wafer by handling the wafer with a grounded conductive resin material. PA1 3 Neutralizing an electric charge in a wafer by handling the wafer with a grounded metallic material. PA1 (1) Generation of corpuscles from a tip of a discharging electrode PA1 (2) Generation of ozone
Results of testing as well as results of computing to identify the trouble 1 are introduced below. FIG. 1 shows a number of particles with the diameter of 0.5 .mu.m or more which adhered to a surface of an electrified wafer when a 5-inch wafer is left on a conductive grating floor for 5 to 10 hours in a clean room in the vertical position with a 2 cm high insulating stand. The horizontal axis shows electric potentials in the wafer, and the vertical axis shows a number of deposited particles (converted to a number of particles which adhered to a central area of a wafer when the wafer is left for 5 hours in the atmosphere with the density of 10 particles with the diameter of 0.5 .mu.m or more /cf). As adhesion of particles due to gravity does not occur on a vertical surface, adhesion of particles is not observed when electric potential of a wafer is in a low range from 0 V-50 V. In accordance with increase of electric potential of the wafer to 300 V or to 1800 V, the number of adhered particles sharply increases, which shows that the adhesion is caused by a static electricity force. FIG. 1 shows a case where effects of static electricity force to relatively large particles were measured, and generally as diameter of a particle becomes smaller, the effects of this static electricity force become visible acceleratively. When electric potential of a wafer is at least 50 V or below, any particle deposits on it. Herein, a state where electric potential of a wafer is 50 V or below is defined as a state where electric potential of the wafer has been neutralized. FIG. 2 shows a range of movement of particles moved and adhered due to static electricity force on an effective section of a wafer calculated on the assumption that electric potential of the wafer is 1000 V and electric potential at the peripheral rectangular frame line is zero. As a force to deposit particles, only gravity (including buoyancy) and static electricity force are taken into account. Also it is assumed that the particle density is 1 g/cm.sup.3. This figure shows that particles in an area enclosed by oblique lines adhere to the effective section of the wafer. Results of the calculation show that an area where particles with the diameter of 2 .mu.m or more adhere to is very narrow, which shows that virtually no particle adheres to the wafer. As the pareticle diameter becomes smaller to 0.5 .mu.m or 0.1 .mu.m, the adhesion area sharply becomes larger, which indicates that, when diameter of a particle is small, the effect of static electricity force over the particle in terms of adhesion to a surface of an object is very large.
Results of the experiment and calculation described above indicates that prevention of electrification of a wafer is very important for preventing a surface of the wafer from being contaminated by particles.
Conventional art for prevention of electrification of a wafer is classified to the following two ways.
All of these techniques have defects which may be fatal in the age of submicron ULSI, and unless these defects are removed, they are not applicable for neutralization of enhanced wafers.
It has turned out that the corona discharge in 1 above has the following problems.
This inventor investigated a cause for (1), and found out that spattering due to ions occurs at a tip of a discharging electrode and corpuscles are generated because of this phenomenon. FIG. 3 shows numbers of ions and corpuscles (.gtoreq.0.17 .mu.m) generated when spark discharge is performed by using a tungsten needle. The numbers of generated ions and corpuscles vary according to strength of loaded discharge current, and when a current value is 1 mA, positive ions are generated at a rate of 200 millions pcs/sec with particles with the diameter of 0.17 .mu.m or more generated at a rate of 1960 pcs/sec. It is conceivable that particles with smaller diameter are generated at a higher rate. As this experiment result shows a case of spark discharge, it is conceivable that a quantity of dust generated in corona discharge would be smaller. But, as spattering, which is the same phenomenon as that in case of spark discharge, occurs, the possibility of dust generation can not be denied.
Then, ozone in (2) is generated when air is electrolytically dissociated, and as ozone's oxidizing effect is very strong, a oxidized film is rapidly formed on a surface of a wafer, which causes various troubles. Also, it has turned out that high polymer materials often used as, for instance, coating material for power cables are dissolved by ozone, which causes many troubles such as insulation fault. Unless these problems are solved, an electrified surface neutralizing method making use of ions generated by means of corona discharge can not be applied for wafers.
In the method 2, a conductive substance mixed with a resin material is a source of contaminants for wafers. Generally carbon or metal is used as a substance to be mixed with. When the substance contacts a wafer, the impurities adhere to the wafer, which causes a dark current or a leak current.
Also in the method 3, like in the method 2, conductive metal contacts a wafer, which may generate a dark current or a leak current (contamination by metal) causing severe contamination, so that the method is not applicable for production of wafers unless it is improved.